The Reynolds stress field of the urban roughness sublayer is studied experimentally at a site in the centre of Zürich (Switzerland). Turbulence observations at various heights within and above a street canyon are used together with profiles of mean variables to determine an average profile of Reynolds stress for the lowest few tens of meters of an urban roughness sublayer.

The spatially averaged net vertical transport of momentum or Reynolds stress is found to be essentially zero at a height close to the average zero plane displacement and increases higher up in the urban roughness sublayer. A parameterisation for the height dependence is provided based on the height above zero plane displacement and a reference friction velocity. Resultes of a quadrant analysis for Reynolds stress indicate that eddies of the organized shear flow are broken up in the vicinity of the zero plane displacement into smaller, less correlated (random) flow patterns. Although not constant with height, turbulent flux of momentum is shown to be the relevant process for the description of the profile of mean wind speed even in the urban roughness sublayer.